This invention relates generally to golf clubs and, in particular, to so-called metal wood golf clubs.
Golf clubs known as “woods” traditionally have a head made of a suitable wooden material such as maple or persimmon attached to one end of an elongated shaft. These traditional wood clubs are usually solid with a striking surface made of wood with or without a reinforcement (e.g., a polymer insert) at the intended impact point. Golf club “wood” heads have also been formed of suitable metals such as stainless steel, aluminum, and titanium. Metal wood heads are usually hollow so as to minimize weight while leaving the maximum amount of material available for the structural components of the heads. When the face of a golf club head strikes a golf ball, large impact forces are produced. In the relatively thin faces of hollow metal wood club heads, these large impact forces produce high shear and bending stresses, primarily in the area of the golf ball impact and around the perimeter of the face where it is joined to the sole, crown and sidewalls that make up the club head. In the extreme, these impact forces can exceed the low cycle fatigue limit of the material or even the ultimate tensile strength of the material, leading to face bending and cracking after an unacceptable short service life.
Various attempts have been made to reinforce the faces of hollow metal woods. Uniformly increasing the thickness of the club face requires the addition of a large amount of material much of which is very lightly stressed during use. The addition of such a large amount of material to a club face, however, adversely affects the performance of the club. The club performance is adversely affected because the club head center of mass is moved too far forward of the shaft axis and the club face is rendered too stiff for optimum energy transfer from the club to the golf ball.
Adding ribs to the back surface of the club face to stiffen the face has the benefit of stiffening without adding a significant amount of weight to the face. Ribs, however, have the detrimental result of causing stress concentrations and, if the ribs are asymmetrical, they may induce non-uniform bending over the surface of the face. Examples of such asymmetrically ribbed club faces include U.S. Pat. No. 5,474,296 to Schmidt, et al. which discloses a hollow metal driver having a front face with a large internal rib extending from the heel toward the middle of the face. U.S. Pat. No. 5,830,084 to Kosmatka discloses a hollow metal driver in which the internal reinforcements are smoothly contoured from the center of the club toward the crown and sole and toward the heel and toe. The ribs disclosed in Kosmatka yield a cruciform reinforcing structure that ties into the center of the crown and sole as well as the center of the side walls. Although the smoothly contoured cruciform reinforcing structure is an improvement over the prior art asymmetric reinforcements and sharply defined ribs, because of the aspect ratio of the golf club, the vertical rib will necessarily be shorter than the horizontal rib. This leads to higher stresses at the face-crown interface and crown-sole interface than at the face-heel and face-toe interfaces. Consequently, the face-crown interface and face-sole interface must be substantially reinforced with additional material that adds weight to the face. Moreover, the shorter, stiffer vertical rib causes the cruciform reinforced face to have asymmetric bending characteristics leading to unpredictable performance. Accordingly, what is needed is a contoured golf club face in which the smoothly contoured stiffening regions are substantially symmetrical and equal in length to provide a reinforced face having uniform properties.